1. Field of Invention
The present invention relates to a planar light source. More particularly, the present invention relates to a planar light source which can increase the lightness in the central portion of the display.
2. Description of Related Art
Because liquid crystal applied in a liquid crystal panel cannot emit light itself, a backlight module has to be added below the liquid crystal panel. The main function of the backlight module is to provide a light source with sufficient lightness and uniform distribution, so that a liquid crystal display (LCD) can show an image. In general, the backlight module may be divided into two types of design, which are a direct-type backlight module and an edge-type backlight module. The design of the direct-type backlight module is to install a light source (e.g. a fluorescent lamp) right below the backlight module and to use more than two lamps to increase the light intensity, and thus is suitable for use in a display panel requiring higher lightness and larger size, such as a liquid crystal monitor or liquid crystal television.
Referring to FIG. 1, FIG. 1 is a schematic perspective view showing a planar light source 10 of a conventional direct-type backlight module. The planar light source 10 includes an upper substrate 20, a lower substrate 30 with a plurality of grooves (not labeled) and a plurality of cold cathode fluorescent lamps 40 arranged in parallel between the upper substrate 20 and the lower substrate 30, wherein the cold cathode fluorescent lamps 40 are installed below a display panel (not shown) for providing sufficient lightness for the display panel. Because the cold cathode fluorescent lamps 40 all are of substantially the same tube diameters, they basically have the same lightness, and additionally, the cold cathode fluorescent lamps 40 are arranged in parallel and evenly spaced to each other for luminous uniformity. In practice, due to the limitation of human vision, the planar light source is merely required to have the luminous uniformity of about 75-80%, wherein too high uniformity actually does not add any extra value to the display quality. Moreover, the human vision generally demands higher lightness in the central portion of the panel. However, the conventional planar light source has quite uniform distribution of lightness, and it has to use a larger power light source installed in the central portion of the display panel for meeting the requirement of higher lightness in the central portion, thus resulting in the problem of the display panel having too much power consumption.